Proceedings of the 29th International Conference on Low Temperature Physics (LT29)
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Proc. 29th Int. Conf. Low Temperature Physics (LT29)
JPS Conf. Proc. 38, 011029 (2023)
https://doi.org/10.7566/JPSCP.38.011029

Pressure Eﬀect on Superconducting and Structural
Transition of LaPd2Al2
Yusuke Hirose1 , Yu Ikeda2 , Yuya Aketa1 , Fuminori Honda3 , and Rikio Settai1
1

Department of Physics, Niigata University, Niigata 950-2181, Japan
Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan3Central Institute of Radioisotope Science and Safety, Kyushu University, Fukuoka 819-0395,
Japan
2E-mail: yhirose@phys.sc.niigata-u.ac.jp(Received July 31, 2022)
We investigated the pressure eﬀects on the structural phase transition and superconduncitivity ofthe tetragonal CaBe2 Ge2 type LaPd2 Al2 , which shows the orthorhombic distortion at T str ∼90 K
and superconducting transition at T c =1.95 K, by measuring the electrical resistance of the strain
gage resistance attached on the sample and sample itself under high pressure. The constructed
temperature−pressure phase diagram from the experimental results revealed that pressure stabilizes
the low temperature orthorhombic structure and suppresses the superconducting state. These experimental results are diﬀerent from the chemical pressure eﬀect expected from the Ga substitution,
which decreases T str and increases T c .
KEYWORDS: single crystal, superconductivity, structural phase transition, CaBe2 Ge2 typestructure, pressure effect1.

Introduction

La-based compounds have been studied as nonmagnetic reference materials to the isostructural
Ce-based compounds that exhibit novel physical properties in the vicinity of the magnetic quantum
critical point (QCP). Nonetheless, some La-based compounds exhibit interesting phenomena such as
the superconductivity (SC) in LaPd2 Si2 and LaPt2 Si2 [1, 2]. Most of LaT2 X2 (T: transition metals,
X: group 12 and 13 elements) compounds crystalize in two types of structures; the ThCr2 Si2 typeand CaBe2 Ge2 type. Both structures are derived from the tetragonal BaAl4 type structure, where La
occupies Ba site while T and X occupy Al sites in diﬀerent ways. Interestingly, LaPt2 Si2 and LaPd2 X2(X=Al and Ga) with the CaBe2 Ge2 type structure show structural phase transition and SC state [2,3].
In La compounds with a structural instability, the correlation between structural phase transition andSC is discussed in terms of the structural QCP [6].
LaPd2 X2 (X=Al and Ga) shows a structural phase transition with an orthorhombic distortion
at T str ∼90 and 50 K and SC transition at T c =1.88 and 1.9 K, respectively [3, 4, 7]. The structuralphase transitions of LaPd2 Al2 and LaPd2 Ga2 are reported to be the second- and ﬁrst-order types,respectively [8]. The previous systematic studies in LaPd2 Al2−x Ga x revealed that the lattice constants
become continuously small as x increases. T str and T c show a minimum and maximum, respectively,
at x=1.6, where the structural phase transition switches from the second- to ﬁrst-order types [3, 5, 8].
These experimental results indicate that the suppression of the second-order structural phase transition leads to the enhancement of T c . In the present study, we measured the electrical resistivity andsample strain under pressure to investigate the pressure eﬀect on T c and T str of LaPd2 Al2 .

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JPS Conf. Proc. 38, 011029 (2023)

2.

Experimental

Single crystals of LaPd2 Al2 were grown by the Bridgman method. The elemental ingots of3N(99.9% pure)-La, powders of 3N-Pd, and shots of 5N-Al were sealed into a Mo crucible witha stoichiometric ratio of La : Pd : Al = 1 : 2 : 2. The crucible was heated up to 1450 ◦ C in an electric
furnace and slowly cooled down to 850 ◦ C. Ingots were obtained in which several small single crystals are combined. The small single crystal pieces were used to measure the electrical resistivity andstrain gage resistance, while an ingot was used to measure the magnetic susceptibility. The electricalresistivity was measured by a standard four-point method for the current perpendicular to the [001]direction in the temperature range of 0.7−300 K. We measured the resistivity of strain gage (Kyowa Co. Ltd.) glued to a sample to detect the structural phase transition. The magnetic susceptibility
measurement was performed using an ingot sample and a magnetic property measurement system(MPMS, Quantum design Co. Ltd.). The pressure was generated in a Bridgman anvil cell (BAC)
and a piston cylinder cell (PCC) to measure the electrical resistivity and sample strain, respectively.
A Daphne 7373 was used as the pressure-transmitting medium for both pressure cells. The appliedpressure was calibrated via superconducting transition of Pb for BAC and the resistivity of manganin
wire at room temperature for PCC, where Pb and manganin wire were placed beside the sample in
the pressure cells.

3.

Results and discussions

3.1

Electrical resistivity
Figure 1(a) displays the temperature dependence of the electrical resistivity ρ of LaPd2 Al2 at several pressures. With decreasing temperature, ρ monotonically decreases with a shoulder-like structure
at about 100 K, which might correspond to T str , and drops to zero at T c =1.95 K at ambient pressure.
Here we deﬁne T str as the characteristic temperature where the temperature derivative of ρ, dρ/dT ,shows the maximum slope, as displayed in Fig. 1(b). T str is estimated to be 95 K, which is in good
agreement with T str determined from the magnetic susceptibility and strain measurements, as discussed later. T c is deﬁned as the temperature where ρ becomes zero, as described by the arrows in theinset of Fig. 1(a). The residual resistivity ratio (RRR) estimated from the value of ρ just above T c of
1.1 µΩcm is about 35, indicating the high quality of samples.
By applying pressure, the value of ρ at room temperature is reduced, and the shoulder-like structure characterizing T str is shifted to higher temperatures, leading to the position shift of the maximum
slope in dρ/dT , as shown in Fig. 1(b). T c gradually decreases to 1.65 K at 4.6 GPa. Although the residual resistivity slightly decreases with increasing pressure, ρ(T ) at measured pressures overlap below
10 K, indicating no signiﬁcant change in the electronic contribution, T 2 term in ρ. The decrease ofT c under pressure might be attributed to the increased stiﬀness of the lattice, as seen in normal metal
superconductors [9].
3.2Magnetic susceptibility and strain gage resistance
The temperature dependence of the magnetic susceptibility χ is shown in Fig. 2(a). χ(T ) showsa small positive value with weak temperature dependence. The dominant contribution of small positive and temperature independent χ might be attributed to the Pauli paramagnetic susceptibility. χ(T )shows a clear cusp at T str ∼ 89 K, which is in good agreement with the value of 91.5 K in the previous neutron study [4]. The decrease of Pauli paramagnetic susceptibility below T str is most likely
due to a reduction of the density of states at the Fermi level. In the case of isostructural LaPt2 Si2 ,which exhibits the structural phase transition accompanying a charge density wave (CDW), χ showsa step-like decrease at T str due to a gap formation in the Fermi surface [10]. The slight variation inχ of LaPd2 Al2 at T str indicates that the structural phase transition might not signiﬁcantly change the2Proceedings of the 29th International Conference on Low Temperature Physics (LT29)
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JPS Conf. Proc. 38, 011029 (2023)

(a)

(b)

(μΩcm)

30

1.00
1.6

200 GPa
Tc

0.5

1.8T (K)

P=

2.010

LaPd2Al2
j [001]00d /dT (a.u.)(μΩcm)

400 GPa
1.01.33.0
3.6
4.5100200Temperature (K)

1.0
1.3
3.0
3.6
4.50300

100200Temperature (K)300Fig. 1. Temperature dependences of (a) the electrical resistivity ρ of LaPd2 Al2 for J perpendicular to the[001] direction and temperature-derivative of ρ at several pressures. The inset of (a) shows an enlarged view ofthe low-temperature region. Solid lines in (b) are guides to the eye.

electronic state, leading to no sign of T str in ρ(T ).
We also detected the structural phase transition for the measurement of the strain gage resistance
R, where the resistance reﬂects a strain in the [110] direction in tetragonal description. The resistanceshows a slope change at T str ∼95 K, which is almost consistent with T str determined from χ(T ). The
previous X-ray diﬀraction experiment revealed that the (220) diﬀraction peak splits into two peaks
below T str , where the (220) and (2̄20) diﬀractions are not equivalent in orthorhombic phase [8]. Consequently, a domain with extending or shrinking [110] direction is formed in orthorhombic phase,
which might lead to the gradual change in R. With increasing pressure, the variation of R at T str becomes broad, and T str is shifted to higher temperatures. The increase in T str under pressure means thestabilization of the lower temperature orthorhombic phase, supporting the scenario described abovethat the increased stiﬀness reduces T c .
0.6

0.51

LaPd2Al2
ε || [110]

0.49
Tstr
0.48
0.47
60

0.4(b)

0.50

80

100
T (K)

120

R (a.u.)0.5-6

M/H (10 emu/g)-6

M/H (10 emu/g)(a)

1.1 GPa

LaPd2Al2μ0H = 1.0 T

Tstr
0 GPa

0100200Temperature (K)30080900.6 GPa

100 110 120 130 140 150
Temperature (K)Fig. 2. (a) Temperature dependence of the magnetic susceptibility of LaPd2 Al2 at 1.0 T. Inset of (a) is magniﬁed view of the temperature near T str . (b) Temperature dependences of the strain gage resistance at several
pressures with guide lines. The data are shifted vertically for the clarity.

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JPS Conf. Proc. 38, 011029 (2023)

Phase diagram
From the present experimental results, a temperature(T )−pressure(P) phase diagram was constructed and plotted in Fig. 3. With the application of pressure, T c decreases almost linearly with aslope of −0.068 K/GPa. On the other hand, T str is enhanced to about 150 K at by applying pressure up
to 4.5 GPa. Assuming a linear relation between T str and pressure, the slope is roughly estimated to be
12 K/GPa described as the solid line in Fig. 3. This pressure eﬀect on T c and T str of LaPd2 Al2 is diﬀerent from the chemical pressure eﬀect expected from the Ga substitution mentioned in Introduction.
The randomness induced by the substitution can be a dominant eﬀect on the suppression of T str in
LaPd2 Al2−x Ga2 . A similar substitution eﬀect is observed in superconductor 2H−TaSe2 , which exhibits a CDW transition In S-doped TaSe2 , the chemical disorder was found to be the dominant eﬀect
on the suppression of T str and enhancement of T c in S doping to TaSe2 [11].

Temperature (K)

3.3

160
140 LaPd2Al212010080604020001Tstr×10 Tc
23Pressure (GPa)45Fig. 3. The obtained temperature−pressure phase diagram of LaPd2 Al2 , where open circles, open and close
squares of T str are determined from measurements of electrical resistivity, magnetic susceptibility and strain
gage resistance, respectively, and T c is deﬁned as the zero resistivity.

In a series of isostructural compounds CePd2 Al2−x Ga x , which also exhibit the structural phasetransition, T str increases and the unit cell volume decreases with increasing Ga content [3, 8]. The x
dependence of T str in CePd2 Al2−x Ga x is diﬀerent from that in LaPd2 Al2−x Ga x , where T str decreases
with increasing x and shows a minimum at x=1.6. On the other hand, the pressure eﬀect on T str ofLaPd2 Al2 is similar to that of CePd2 Ga2 [12], where T str linearly increases as a function of pressure
with the slope of 28 K/GPa. The enhancement of T str in CePd2 Ga2 under pressure is discussed interms of the c/a ratio. The ratio is suggested as a criterion for structural instability in CaBe2 Ge2type structure [7], where the value of c/a smaller than 2.3 indicates the instability. However, thiscriterion can not explain the opposite reaction of T str on x in LaPd2 Al2−x Ga x and CePd2 Al2−x Ga x ,since the c/a ratio of both systems decreases with increasing Ga content [3, 8]. In order to elucidate
the mechanism of the structural phase transition and relation between T str and c/a ratio in this system,
the X-ray diﬀraction experiments under pressure are desirable.

4.

Summary

We investigated the pressure eﬀects on T c and T str of LaPd2 Al2 by measuring the electrical resistivity and strain gage resistance under pressure. The constructed T − P phase diagram revealed4

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JPS Conf. Proc. 38, 011029 (2023)

that pressure stabilizes the orthorhombic phase and suppresses the superconducting state. No significant variation of electrical resistivity at measured pressures in the temperature range below 10 K isobserved, indicating that a dramatic change in electronic state does not occur under pressure. The
gradual decrease of T c might be attributed to an increase in lattice stiﬀness, which is consistent withthe stabilization of the lower temperature phase.
These experimental results are diﬀerent from the chemical pressure eﬀect in LaPd2 Al2−x Ga x .
The Ga substitution can not be regarded as a simple chemical pressure eﬀect. In order to realize thestructural QCP in LaPd2 Al2 , another approach should be investigated.

Acknowledgement
This work was ﬁnancially and technically supported by JSPS KAKENHI Grant Numbers JP20K14414,
JP19K03713, and JP19K21840, and CCRF, Niigata University and Low temperature center, Kyushu
university, respectively.
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